1. Field of the Invention
This invention relates generally to a system for pumping fluid from a well. More specifically, this invention relates to a system in which a dual-displacement, subsurface pump is driven by reciprocating motion of a sucker rod or tubing string, thereby producing fluid with both halves of the stroke cycle.
2. Description of Related Art
To extract fluids such as water or hydrocarbons from the earth, people traditionally drill a hole through overlying formations to the fluid-containing reservoir. If the fluid pressure in the reservoir is sufficient, the fluids will fill the hole and flow to the surface of their own accord. More commonly, however, fluids will enter the hole and remain pooled near the bottom. These fluids must be pumped to the surface.
Of particular interest to this disclosure are wells with high water/oil ratios and high fluid volumes. These may occur, for example, in secondary recovery oil wells where water is injected to xe2x80x9csweepxe2x80x9d the last traces of hydrocarbons from a reservoir.
A popular pumping system for these wells includes an electric submersible pump. In this system, the pump is typically attached to the lower end of production tubing and submerged in the fluid. An electrical cable is typically attached to production tubing to supply power for the pump. However, for deeper wells, the installation of pumping system becomes cumbersome, requiring manual strapping of the cable to the production tubing, and careful insertion to avoid accidental severing of the cable downhole. Once in place, the power dissipation in the cable may become a significant portion of operational costs.
For most wells of this type, the traditional pumping system includes a single-displacement reciprocating pump. The pump is typically attached to the lower end of production tubing and submerged in the fluid. A sucker rod string extends through the production tubing between the pump and a surface pump unit on the surface. The surface pump unit reciprocates the sucker rod string to drive the single-displacement pump. Although reliable, this pumping system generally requires a large surface pumping unit, and it productively utilizes only one half of the pumping cycle.
An alternative pumping system that is sometimes employed for these wells is a progressive-cavity pumping system. In this system, a progressive-cavity pump is attached to the lower end of a sucker rod string and inserted through production tubing to be submerged in the fluid. The sucker rod string connects the pump to a surface pump unit. The surface pump unit rotates the sucker rod string to drive the progressive cavity pump. Although these pumps can be run at high speed, such operation commonly causes failure in the sucker rod string. This failure is normally attributed to improper installation and/or inertial torque stresses. These systems are also subject to depth limitations.
Accordingly, a need exists for a pumping system that can operate reliably and more economically than existing pumping systems.
The problems outlined above are addressed by a dual-displacement pumping system. In one embodiment, the system includes a dual-displacement pump, a tubing string, and a surface pumping unit connected to the dual displacement pump by a reciprocating member. The reciprocating member is preferably a continuous tubing string, but a threaded tubing string or a sucker rod string with a hollow portion at its terminal end may alternatively be used. The dual displacement pump includes a pump barrel mounted to the end of the first tubing string, and a plunger mounted to the end of the reciprocating member. A valve configuration is provided so that downward motion of the plunger in the pump barrel forces fluid from the lower end of the pump barrel to enter the reciprocating member and from there, to travel to the surface. Downward motion of the plunger also fills the upper end of the pump barrel with fluid from the well bore. The valve configuration also causes upward motion of the plunger to force fluid from the upper end of the pump barrel to enter the tubing string (and travel thence to the surface), and causes the lower end of the pump barrel to fill with fluid from the well bore. In this fashion, both movements of the pumping cycle are fully exploited to nearly double the volume of fluid pumped with a conventional surface configuration.